This invention is directed to a process for preparing a high quality lubricant bright stock from heavy petroleum feedstocks. These heavy feedstocks are often contaminated with sulfur, nitrogen, asphaltenic and metal contaminants, which must be removed in preparing the lubricant base stock. They also generally contain significant amounts of waxy materials.
Low valued oils such as deasphalted oil (DAO) are increasingly being hydrotreated and used as FCC cracker feed to produce gasoline. Severity of deasphalting is much less for making fuels than for making lubricant bright stock. Consequently, the purity of fuels-application DAO is too low to make lubricant bright stock with adequate stability for use in finished lubricant applications. However, increasingly stringent mandated limits on gasoline sulfur require higher severity in fuels DAO hydrotreating. These changes improve the quality of DAO as a feed to hydrocracking to produce high viscosity lubricant base oils. Nevertheless, the highest boiling portion of DAO contains high molecular weight waxes which are difficult to remove, leading to low yields of high cloud point products. The highest boiling fractions also contain large polycyclic molecules, which are difficult to completely saturate in hydrofinishing and which lead to stability problems.
Conventional high quality Group II lubricant neutral oils having excellent oxidation stability, good low temperature properties and high viscosity indices are generally made by hydrocracking gas oils, followed by dewaxing and optionally mild hydrotreating. Lubricant base stocks having viscosities of up to about 100 cSt, measured at 40° C., are made in this manner. Higher viscosity oils, for example, bright stocks and similar oils with viscosities of 220 cSt or greater, are generally not made by isomerization dewaxing and generally do not posses the high quality of Group II base oils prepared by isomerization dewaxing. High viscosity oils of improved quality are in general demand, especially for non-engine oil applications, such as industrial oils.
Such high viscosity oils generally require some bright stock in their formulation, the amount of which depends on the product. In typical formulations, Group I bright stock is used, which degrades the product when blended with neutral oil. One problem with the quality of bright stock is that it is not a distillate and is typically of low quality, particularly with respect to oxidation stability. Thus, there is a need for a method for producing an oxidation stable, good quality lubricant bright stock.
In addition, feedstocks which are useful for making lubricant bright stock have generally been limited to gas oils, and specifically vacuum gas oils. Residuum streams are generally difficult to process for lubricant base oils. Not only are the sulfur, nitrogen and aromatic contents of residuum streams very high, but the waxy materials present in these residuum streams are difficult to process in the production of low pour point base oil products. It is especially, therefore, to be able to produce good quality lubricant bright stock from residuum-derived streams.